1. Field of the Invention
The present invention relates to a discharging lamp lighting apparatus for performing lighting control of high voltage discharging lamps such as high pressure sodium lamp or metal halide lamp, for use in headlamps of vehicles.
2. Description of the Prior Art
A high voltage discharging lamp is a lamp into which vapor of metal such as halogenated metal or sodium is sealed as light emitting material so as to use discharge for light emission. In the high voltage discharging lamp, high voltage is applied across electrodes when starting to light. The high voltage discharging lamp is widely used for headlamps of vehicles because of facilitation of small-sized design thereof, and high efficiency.
However, there is a drawback in that a time of about several seconds is required for reaching a stable state of discharge when starting to light. Further, if a control constant at a time when starting to light is set so as to reach a desired amount of light for a shorter time interval, the amount of light becomes excessive when reaching the stable state of discharge. Thus, there is another drawback in that life of the discharging lamp is reduced.
FIG. 1 is a circuit block diagram illustrating a conventional discharging lamp lighting apparatus which can overcome the problems, and is disclosed in, for example, Japanese Patent Application Laid-Open No. 2-215,090. In FIG. 1, reference numeral 2 means an inverter circuit for converting DC current into AC current at a predetermined frequency, 4 means LC series resonance circuit for applying high voltage to a discharging lamp 5, including a choke coil L and capacitors C.sub.1, C.sub.2, 21 is AC power source serving as an energy supplying source, 22 is a full wave rectifying circuit for performing full wave rectification with respect to AC voltage from the AC power source 21, and 23 is a control section for outputting a signal S.sub.1 which controls the inverter circuit 2. Further, a resistor R connected to the discharging lamp 5 serves to detect discharge current value flowing through the discharging lamp 5 as voltage value.
A description will now be given of the operation. When a light switch (not shown) for instructing ON and OFF of the discharging lamp 5 is turned ON, the control section 23 is started to operate so as to set a frequency of the signal S.sub.1 to 100 kHz. The inverter circuit 2 generates AC power of 100 kHz according to the frequency of the signal S.sub.1. Subsequently, the AC power is supplied to the LC series resonance circuit 4.
The LC series resonance circuit 4 generates high voltage of on the order of 10 kV, and the high voltage is applied to the discharging lamp 5. Further, the high voltage causes dielectric breakdown in the sealed gas of the discharging lamp 5. Since voltage is developed across the resistor R due to discharge current generated by the occurrence of the dielectric breakdown, the control section 23 can recognize a time point of the occurrence of the dielectric breakdown by detecting the voltage.
When the control section 23 recognizes the occurrence of the dielectric breakdown, the control section 23 is operated to set the frequency of the signal S.sub.1 to a low value of, for example, 4 kHz so as to increase a discharge current value. Thus, voltage of 4 kHz is applied to the LC series resonance circuit 4. As the frequency is increased, integral value of the current is reduced by the choke coil L of the LC series resonance circuit 4 so that current flowing through the discharging lamp 5 is reduced. That is, if the frequency is reduced, the discharge current is increased to reduce a time required for the discharging lamp 5 reaching the stable state.
However, the discharge current in the stable state is further increased if the frequency is left, and the amount of light is kept excessive. Hence, the control section 23 is operated to set the frequency of the signal S.sub.1 to a predetermined value of, for example, 10 kHz to control the discharge current so as to generate, at a predetermined time point, a condition where the discharge current according to the desired amount of light can flow through the discharging lamp 5.
In this case, if the frequency is rapidly varied from 4 kHz to 10 kHz, the amount of light is also varied rapidly. Consequently, when the discharging lamp 5 is used for the headlamp of the vehicle, the discharging lamp 5 may dazzle a passerby or a driver of another vehicle. Hence, the control section 23 is operated to perform the following control so as to gradually vary the amount of light.
That is, the control section 23 is operated to determine whether or not a time of 100 ms has elapsed, and is operated to increase the frequency of the signal S.sub.1 by 50 Hz each time when the time of 100 ms is elapsed. After the frequency of the signal S.sub.1 reaches 6 kHz, the control section 23 is operated to increase the frequency of the signal S.sub.1 by 100 Hz each time when the time of 100 ms is elapsed. Finally, when reaching 10 kHz, the frequency is fixed to the value of 10 kHz.
As set forth before, it is possible to enhance rise characteristics of the amount of light, and overshadow the variation in the amount of light.
Since the prior art discharging lamp lighting apparatus is provided as described hereinbefore, a final value of the frequency of the signal S.sub.1 is a fixed value of, for example, 10 kHz. Thus, in case, for example, a time elapsing variation occurs in characteristics of the discharging lamp 5 or the mounted discharging lamp 5 is exchanged, there are drawbacks in that the amount of light may be excessive depending upon the variation or a new discharging lamp 5, and the life of the discharging lamp 5 is reduced.